At high pressure and low temperature water within pipelines can form gas hydrates as a result of interaction with low molecular weight gases present, such as methane and ethane. Such gas hydrates can form plugs and result in lost production (and revenue) and cause safety issues in the oilfield industry. Amelioration of such problems may be undertaken by dehydration, heating (such as direct electrical heating) and chemical inhibition of the hydrate formation. Thermodynamic inhibitors (THIs) such as methanol and ethylene glycol are frequently used but require large volumes for treatment so are generally undesirable at least in terms of cost. This high volume requirement relating to THIs can also be particularly problematic for offshore deepwater production.
Low dose hydrate inhibitors (LDHIs) have been developed and applied in oil and gas production as an alternative to the THIs mentioned above. These LDHIs can be divided into two broad subclasses; kinetic hydrate inhibitors (KHIs), and anti-agglomerants (AAs). KHIs act mainly as gas hydrate anti-nucleation compounds (although they may also inhibit gas hydrate crystal growth). Typically KHIs operate at lower subcooling values than AAs. “Subcooling” is the difference between the operating temperature and the temperature at which hydrates first start to form (the hydrate equilibrium temperature) at a given pressure. Higher values of subcooling means that the compounds are able to inhibit hydrate formation at lower temperatures (for a given pressure). KHIs are normally considered to provide time limited protection at subcooling values to around 12° C.
Anti-agglomerants (AAs) can function at higher subcoolings. In contrast to KHIs they allow gas hydrates to form but generate a slurry of dispersed hydrate particles in the liquid hydrocarbon that remains mobile and does not result in a plug forming.
The first AAs were developed by Shell based on quaternary ammonium compounds as described in WO 95/17579 and WO 96/34177. Other chemistries have been described as providing effective AAs, mostly based on quaternary ammonium/phosphonium based molecules (Kelland, M. A., Energy & Fuels, 2006, 20, 825-847 and Kelland, M., Production Chemicals for the Oil and Gas Industry; CRC Press, Boca Raton, Fla., 2009). There is typically a limitation on the fraction of water present in the liquid phase—above a certain amount the hydrate droplets will not flow within the oil phase, the resulting slurry being too viscous (Sloan, E. D. et al., Natural Gas Hydrates in Flow Assurance, Gulf Professional Publishing, Burlington, Mass., 2011, and Anklam, M. A. et al., AIChE Journal, 2008, 54, 565-574). Many AA LDHIs are also limited in that they often exhibit relatively poor environmental compatibility.
WO 2005/116399 and U.S. Pat. No. 6,596,911 describe onium (such as quaternary ammonium) LDHI AA compounds which may include heteroatoms in the alkyl chains (especially the longest chain) attached to the central atom.
U.S. Pat. Nos. 7,183,240 and 8,034,748 describe LDHI AA compounds based on a quaternary ammonium structure in which the chains attached to the central atom contain alkylene oxide units.